It's the infrared sensor for the Enhanced Flight Vision System (EFVS).
From FedEx Newsroom June 23, 2008:
FedEx Express is the first major commercial carrier in the airline industry to receive a Supplemental Type Certificate from the FAA for the advanced system, authorizing its installation in the company’s fleet of Boeing MD-10 freighters. The company’s goal for the system is to improve the level of flight safety by increasing visibility of pilots during adverse weather conditions and darkness.
A unique application of Honeywell International “Head Up Display” (HUD) technology combined with infrared “Enhanced Flight Vision System” (EFVS) technology of Elbit Systems of America Commercial Aviation-Kollsman Business Unit, positions the system as the leader in the avionic visual technologies market.
The Honeywell HUD interfaces with aircraft navigational and flight data systems in presenting a high resolution liquid crystal display of critical flight guidance information. This is overlaid with real-time EFVS infrared video of the outside world that is displayed in an overhead unit in the captain’s forward field of view using state-of-the-art HUD “combiner” technology. Elbit Systems Ltd., Electro-Optics-Elop manufactures the combiner glass.